This invention relates to circuits for the control of the frequency of a sine wave oscillator, particularly circuits employing a phase lock loop to achieve such control.
In a variable frequency sine wave signal source it is desirable to produce a sine wave output signal with minimal distortion, to accurately establish and maintain the frequency of the signal, and to achieve frequency stability after adjustment of the frequency as rapidly as possible. Oscillators of various topographies, particularly state variable, resistor-capacitor oscillators, can be made to produce a high quality, that is, low distortion, sine wave output; however, the frequency of such an oscillator is not especially stable, it being affected by variations in component characteristics due to temperature changes. One way to accurately establish and maintain the frequency of such an oscillator is to provide a phase lock loop control circuit which adjusts the frequency of the oscillator relative to the frequency of a reference signal whose waveform is rectangular, rather than sinusoidal, but whose frequency is highly stable. Oscillator frequency control circuits of this type are commonly known, the oscillator frequency being voltage controlled in response to the integral of the phase difference between the oscillator output signal and reference frequency signal.
A problem encountered with phase lock loop controlled oscillators is that at low frequencies, for example, 10 hertz, the time required for the phase lock loop to lock the oscillator signal in phase with the reference signal may be substantial. The time required to achieve lock increases as the tuning range of the oscillator control circuit decreases, yet a limited tuning range is desirable because the frequency modulation noise introduced in the oscillator output due to noise in the phase lock loop increases as the tuning range of the control circuit increases. Hence, there is a need for a method of oscillator frequency control and implementation circuitry that will quickly achieve a phase lock between the oscillator output signal and a reference frequency signal.